The present invention relates to a buried heterostructure semiconductor laser device (BH-LD).
A prior art BH-LD may be fabricated by any of various techniques. The shape of a resulting striped region, including an active layer, differs depending on what technique is specifically employed. In a conventional BH-LD exemplified below, a striped region is formed by dry-etching, and then semiconductor layers are re-grown over the striped region by a metalorganic vapor phase epitaxy (MOVPE) process to bury the striped region therein. Such a BH-LD is described in IEEE Photonics Technology Letters Vol. 8 (1996), pp. 989-991, for example.
Hereinafter, the conventional BH-LD disclosed in this prior art reference will be described with reference to FIG. 7.
FIG. 7 illustrates a cross section of the BH-LD taken vertically to the direction in which the striped region thereof extends. The BH-LD is fabricated in the following manner. First, as shown in FIG. 7, epitaxial layers, namely, n-type InGaAsP first waveguide layer 102, InGaAsP active layer 103, p-type InGaAsP second waveguide layer 104 and p-type InP cladding layer 105 are grown in this order by MOVPE, for example, on an n-type InP substrate 101. Next, a striped mask pattern (not shown) is formed on the upper surface of the uppermost epitaxial layer. Then, these epitaxial layers are dry-etched with a mixed gas of methane and hydrogen using the mask pattern as an etching mask, thereby forming a striped region 106 including the active layer 103.
Then, respective burying layers, namely, p-type InP current blocking layer 107, n-type InP current blocking layer 108, p-type InP semiconductor layer 109 and p-type InGaAsP contact layer 110 are grown in this order over the substrate 101 by MOVPE again so as to cover the striped region 106.
Subsequently, p- and n-side electrodes 111 and 112 are formed by an evaporation technique on the upper surface of the p-type contact layer 110 and on the back of the substrate 101, respectively. The p-side electrode 111 is made up of Au and Zn layers alternately stacked, while the n-side electrode 112 is made up of Au and Sn layers alternately stacked.
The conventional BH-LD, however, has the following drawbacks.
Firstly, each side face of the striped region 106 has a substantially uniform crystallographic plane orientation defined between (0-11)B and (1-11)B, and receives some damage caused by the dry-etching during the re-growth process for burying the striped region 106 in the epitaxial layers. Thus, in such a structure, a p-type dopant (e.g., Zn), introduced into these burying layers during their growth on the sides of the striped region 106, is more likely to diffuse over a distance of about 0.2 xcexcm and ultimately reach the active layer 103. In such a case, the long-term reliability of the laser device is seriously affected. It should be noted that a Miller index with a negative sign is supposed to represent a negative direction index in this specification.
Secondly, in the conventional BH-LD, the sides of the striped region 106 are subjected to some surface treatment, like wet etching, to suppress the diffusion of the dopant and thereby ensure long-term reliability. However, since the surface treatment is conducted, the width of the active layer 103, greatly affecting the electrical and optical characteristics of the device, deviates from that defined by the mask.
An object of the present invention is ensuring long-term reliability for a buried heterostructure semiconductor laser device.
To achieve this object, according to the present invention, the side faces of at least one semiconductor layer under an active layer in a striped region have a crystallographic plane orientation defined as (h-11)B, where h is an integer equal to or larger than 1.
Specifically, a first exemplary semiconductor laser device according to the present invention includes: a striped region formed on a substrate; and a plurality of burying semiconductor layers formed on the sides and upper surface of the striped region. The striped region includes lower and upper striped regions. The lower striped region includes a first waveguide layer, while the upper striped region includes an active layer and a second waveguide layer formed on the active layer. The sides of the upper striped region have a plane orientation approximately represented as (0-11) and the sides of the lower striped region have a plane orientation approximately represented as (h-11)B, where h is an integer equal to or larger than 1.
In the first semiconductor laser device, the sides of the lower striped region have a plane orientation approximately represented as (h-11)B. Thus, while the striped region is being buried in the semiconductor layers, crystals are growing mainly in a direction defined between [1-11]B and [100] on the sides of the lower striped region. Thus, crystals also grow in a similar direction on the sides of the upper striped region, too.
In Journal of Crystal Growth, Vol. 107, (1991), pp. 772-778, it is reported that Zn, or a typical p-type dopant, diffuses toward crystals much less in the (h-11)B plane than in the (h-11)A plane (where h is an integer equal to or larger than 1). In the semiconductor laser device of the present invention, the sides of the striped region to be buried in the semiconductor layers have a crystallographic plane orientation defined between (1-11)B and (100). Accordingly, it is possible to minimize the diffusion of the dopant from the burying layers toward the striped region including the active layer, thus preventing the diffusing dopant from deteriorating the active layer. As a result, improved long-term reliability can be easily ensured for the device.
A second exemplary semiconductor laser device according to the present invention includes: a striped region formed on a substrate; and a plurality of burying semiconductor layers formed on the sides and upper surface of the striped region. The striped region includes lower and upper striped regions. The lower striped region includes a first waveguide layer, while the upper striped region includes an active layer and a second waveguide layer formed on the active layer. The sides of the upper striped region have a plane orientation approximately represented as (h-11)B, where h is an integer equal to or larger than 1. The sides of the lower striped region have a plane orientation approximately represented as (j-11)B, where j is also an integer equal to or larger than 1.
In the second semiconductor laser device, the sides of the upper striped region have a plane orientation approximately represented as (h-11)B and the sides of the lower striped region have a plane orientation approximately represented as (j-11)B. Accordingly, it is possible to minimize the diffusion of the dopant from the burying layers toward the striped region including the active layer. As a result, improved long-term reliability can be easily ensured for the device.
In one embodiment of the present invention, a region under the striped region may be of a first conductivity type. The burying layers may include first and second semiconductor layers, which are formed in this order on the sides of the striped region. The first semiconductor layer may be undoped and the second semiconductor layer may be of a second conductivity type.
In such an embodiment, it is possible to prevent the dopant of the second conductivity type in the second semiconductor layer from diffusing toward the striped region with more certainty.
In another embodiment, a dopant concentration in the second semiconductor layer preferably has such a profile that a concentration in a region of the second semiconductor layer is lower than that in another region of the second semiconductor layer, where the former region is closer to the first semiconductor layer than the latter region is.
In such an embodiment, the dopant of the second conductivity type in the second semiconductor layer is even less likely to diffuse toward the striped region.
In still another embodiment, a region under the striped region may be of a first conductivity type. The burying layers may include first and second semiconductor layers, which are formed in this order on the sides of the striped region. The first semiconductor layer may be undoped, while the second semiconductor layer may be of the first conductivity type.
In such an embodiment, since the sides of the striped region are covered with the first semiconductor layer of the first conductivity type, it is possible to prevent the dopant of the second conductivity type in the second semiconductor layer from diffusing toward the striped region with more certainty.
In still another embodiment, a region under the striped region may be of a first conductivity type. The burying layers may include first and second semiconductor layers, which are formed in this order on the sides of the striped region. The first semiconductor layer may be of a second conductivity type, while the second semiconductor layer may be of the first conductivity type.
In such an embodiment, the first and second semiconductor layers function as respective carrier blocking layers with a lot more certainty.
In still another embodiment, a region under the striped region may be of a first conductivity type. The burying layers may include first and second semiconductor layers, which are formed in this order on the sides of the striped region. The first semiconductor layer may be of the first conductivity type, while the second semiconductor layer may be of a second conductivity type.
In still another embodiment, a region under the striped region may be of a first conductivity type. The burying layers may include first and second semiconductor layers, which are formed in this order on the sides of the striped region. The first and second semiconductor layers may be of a second conductivity type. The first semiconductor layer preferably has a dopant concentration lower than that of the second semiconductor layer.
In such an embodiment, it is possible to prevent the dopant of the second conductivity type in the second semiconductor layer from diffusing toward the striped region with certainty. In addition, the first and second semiconductor layers can block the carriers of the first conductivity type with a lot more certainty.
A method for fabricating a semiconductor laser device according to the present invention includes the steps of: a) forming striped-region-forming layers by depositing a first waveguide layer, an active layer and a second waveguide layer in this order on a substrate; b) forming an upper striped region including the active layer by selectively forming a striped mask pattern on the striped-region-forming layers and then etching anisotropically the striped-region-forming layers using the mask pattern in such a manner as to expose both sides of the active layer; c) forming a lower striped region, including the first waveguide layer, under the upper striped region and along the sides of the upper striped region by etching the striped-region-forming layers as well as the upper striped region using the mask pattern, the sides of the lower striped region having a plane orientation approximately represented as (h-11)B, where h is an integer equal to or larger than 1; and d) forming a plurality of semiconductor layers over the substrate to bury the upper and lower striped regions therein.
In the method according to the present invention, the upper striped region, including the active layer, and the lower striped region, including the first waveguide layer, with a plane orientation approximately represented as (h-11)B (where h is an integer equal to or larger than 1) on the sides thereof, are formed. Then, a plurality of semiconductor layer are formed over the substrate to bury the upper and lower striped regions therein. Thus, crystals are growing mainly in a direction defined between [111]B and [100] on the sides of the lower striped region. Accordingly, crystals also grow in a similar direction on the sides of the upper striped region, too. As a result, the first semiconductor laser device of the present invention can be fabricated with certainty.
In one embodiment, the method of the present invention preferably further includes the step of selectively wet-etching the upper and lower striped regions to substantially equalize the plane orientations on the sides of the upper and lower striped regions with each other between the steps c) and d).
In such an embodiment, the second semiconductor laser device of the present invention can be fabricated with certainty.